2008-Biotreatment of indoor air for VOC removal

Biotechnology Advances 26 (2008) 398–410

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Biotechnology Advances

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Research review paper

Biological treatment of indoor air for VOC removal: Potential and challenges

Benoit Guieysse a,⁎, Cecile Hort b, Vincent Platel b, Raul Munoz c, Michel Ondarts b, Sergio Revah d

a School of Civil and Environmental Engineering, Nanyang Technological University, Block N1, Nanyang Avenue, Singapore 639798, Singaporeb Laboratoire de Thermique, Energétique et Procédés, Université de Pau et des Pays de l'Adour, Quartier Bastillac, 65000 Tarbes, Francec Valladolid University, Department of Chemical Engineering and Environmental Technology, Paseo del Prado de la Magdalena, s/n, Valladolid, Spaind Universidad Autónoma Metropolitana-Cuajimalpa c/o UAM-I, Departamento de Ingeniería de Procesos e Hidraúlica, Av. San Rafael Atlíxco No. 186, Col. Vicentina, C.P. 09340, México,Distrito Federal, Mexico

⁎ Corresponding author. Tel.: +65 6790 5282; fax: +65E-mail address: [email protected] (B. Guieysse)

0734-9750/$ – see front matter © 2008 Elsevier Inc. Aldoi:10.1016/j.biotechadv.2008.03.005

A B S T R A C T
A R T I C L E I N F O
Article history:
There is nowadays no single Received 17 December 2007Received in revised form 25 March 2008Accepted 29 March 2008Available online 8 May 2008
Keywords:BiofiltrationIndoor air qualitySick-building syndromeVolatile organic compoundGreen building

fully satisfactory method for VOC removal from indoor air due to the difficultieslinked to the very low concentration (μg m−3 range), diversity, and variability at which VOCs are typicallyfound in the indoor environment. Although biological methods have shown a certain potential for thispurpose, the specific characteristic of indoor air and the indoor air environment brings numerous challenges.In particular, new methods must be developed to inoculate, express, and maintain a suitable and diversecatabolic ability under conditions of trace substrate concentrationwhich might not sustain microbial growth.In addition, the biological treatment of indoor air must be able to purify large amounts of air in confinedenvironments with minimal nuisances and release of microorganisms. This requires technical innovations,the development of specific testing protocols and a deep understanding of microbial activities and themechanisms of substrate uptake at trace concentrations.

© 2008 Elsevier Inc. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3982. Indoor air quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3993. VOCs and indoor air quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3994. Indoor air treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4005. Biological treatment of indoor air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402

5.1. Influence of type of VOCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4035.2. Influence of low concentrations on biomass productivity and transfer rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4035.3. Impact of purification efficiency on design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4055.4. Humidification and biohazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4075.5. Esthetic, noise, purification perception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4075.6. Evaluating performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407

6. Designing biological purifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4077. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408

1. Introduction

Indoor air contamination is a complex problem involving particles(such as dust and smoke), biological agents (molds, spores), radon,

6791 0676..

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asbestos, and gaseous contaminants such as CO, CO2, NOx, SOx,aldehydes and VOCs (Volatile Organic Compounds) (Table 1). Thelatter are strongly suspected to cause many Indoor Air Quality (IAQ)associated health problems and “sick-building” symptoms (Wallace,2001; Jones, 1999; Wieslander et al., 1997; Yu and Crump, 1998).

Singularly, despite the abundance of evidence linking the exposureto VOCs in indoor air with various health effects, only few reportsevaluating the existing abatement technologies are currently

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Table 1Typical air pollutants in indoor air (US EPA, 2008)

Group Definition/example Origin Toxic effects

Particles Very small liquid or solid substances in suspension inthe air: mists, dust, pollen, cigarette smoke, viruses,bacteria, molds

Outdoor air, combustion, carpets, humanactivity, decaying building

Irritation to eyes and/or respiratory tissues, allergies, cancer,indirect effect through biological production of toxins.

Gaseouspollutants

CO, CO2, NOx, formaldehyde, VOCs Combustion, human activity, buildingmaterials, furniture, cleaning products, molddevelopment etc.

Irritation to eyes and/or respiratory tissues, allergies, cancer,effects on the respiratory liver, immune, reproductive and/ornervous system

Radon andits progeny

Radioactive gases Rock, soil, groundwater, natural gas, mineralbuilding materials

Lung cancer

399B. Guieysse et al. / Biotechnology Advances 26 (2008) 398–410

available. Several studies have demonstrated the potential ofbiological methods to remove indoor VOCs (Wolverton et al., 1984;Wolverton et al., 1989; Darlington et al., 2000; Darlington et al., 2001;Chen et al., 2005; Orwell et al., 2006; Wood et al., 2006). However,there is still a lack of solid and relevant data available to understandthe true removal mechanisms in these systems and the apparentmismatch between experimental observations and theoretical resultsfrom transfer-based models on air biological treatment, as demon-strated in this review. This paper presents a critical review on thepotential of biological technologies for indoor air purificationwith thedual objective of providing a state of the art of the relevant literatureand a roadmap for future research. For the latter purpose, a scale-down approach was used in order to understand the consequences ofindoor biological treatment on microbial growth and process design.Then, we identified current knowledge gaps hindering the properunderstanding and development of biological processes for indoor airVOC removal. In the following, the term “biological purifier” is used todescribe any device including a biological component (botanical ormicrobial) used for VOC removal; “botanical purifier” was used tospecifically describe devices using plants and their associatedmicroorganisms. Classical systems were named according to theconventional air treatment nomenclature (Revah and Morgan-Sagastume, 2005).

2. Indoor air quality

Because Americans spend nearly 90% of their time indoors andnearly 25% of US residents are affected by poor IAQ either at theworkplace or at home, the US Environmental Protection Agency (EPA)ranks poor IAQ among its largest national environmental threats. Itscounterpart, the European Environmental Agency (EEA) has pointedout IAQ as one of the priority concerns in children's health and similarissues are faced worldwide (Zhang and Smith, 2003; Observatory onIndoor Air Quality, 2006, Zumairi et al., 2006). In fact, some buildingscontain such high levels of contaminants that they are qualified as“sick” because exposure to them results inmultiple sickness symptoms(e.g. headache, fatigue, skin and eye irritations, or respiratory illness)commonly described as the “sick-building syndrome” (SBS) (Burge,2004).

Data on IAQ-health related effects is still lacking and sometimescontradictory. For instance, Pilotto et al. (1997) found a strong linkbetween exposure to peak NO2 concentration and respiratoryinfections in children aged 6–10 years whereas other authors failedto establish this association (Brunekreef et al., 1990; Samet et al.,1993). Nevertheless, it has been suggested that indoor air pollutioncauses between 65,000 and 150,000 deaths per year in the US, whichis comparable to outdoors pollution induced mortality (Lomborj,2002). IAQ also impacts work productivity as for instance Wargockiet al. (1999) showed subjects exposed to a typical indoor pollutionsource (plastic carpet) typed 6.5% less than control subjects. Likewise,empirical studies have shown that the use of ventilation rates lowerthan 25 L s−1 per person in commercial and institutional buildings wascorrelated to an increase in the number of short-term sick leaves

(Sundell, 2004). It has therefore been estimated that 40–200 billionsUSD could be annually saved or gained by improving IAQ in the USAonly (in 1996 USD; Fisk, 2000). This problem is already driving animportant IAQ market that reached $5.6 billion in 2003 in the USAwhere it was expected to rise up to $9.4 billion by 2008 (Marketreport: indoor air quality, 2004).

3. VOCs and indoor air quality

Interestingly, there is no clear or unanimous definition of what is aVOC: The US EPA defines VOCs as substances with vapor pressuregreater than 0.1 mmHg, the Australian National Pollutant Inventory asany chemical based on carbon chains or rings with a vapor pressuregreater than 2 mm Hg at 25 °C, and the EU as chemicals with a vaporpressure greater than 0.074 mm Hg at 20 °C. Chemicals such as CO,CO2, CH4, and sometimes aldehydes, are often excluded. In addition,sub-classifications such as Very Volatile Organic Compounds (VVOCs)or Semi Volatile Organic Compounds (SVOCs) have been used in thecontext of IAQ (Crump, 2001; Ayoko, 2004).

Several organizations such as the World Health Organization(WHO), the US EPA, or the OQAI (French Indoor Air QualityObservatory), have established lists of priority indoor air pollutants(WHO, 2000; Johnston et al., 2002; Mosqueron and Nedellec, 2002,OQAI) based on the ubiquity, concentration and toxic effect of thesubstances involved. These lists are relatively similar and system-atically include aldehydes, aromatics, and halogenates as well asbiocides. Differences are due to the type of pollution taken intoaccount (only chemicals for the EPA, no mixtures such as tobaccosmoke for the OQAI) and the geographic specificities of indoor airpollution. Indeed, variations in the building materials, cleaningproducts, or type of ventilation used generate differences in theindoor air pollution (Sakai et al., 2004). These priority lists will mostlikely evolve upon new analytical and toxicological findings morerelevant to IAQ such as the health effects of chronic exposure tomultiple pollutants at low concentration (Mosqueron and Nedellec,2002). This lack of relevant knowledge probably explains why thereare only few guidelines for VOC indoor air concentration currentlyavailable (WHO, 2000; Canada, 1987).

Hundreds of VOCs can be simultaneously found in indoor air. Thesecompounds exhibit very large variations in concentration as well asphysical, chemical, and biological properties. Furthermore, thecomposition of the mixture greatly varies in time as the concentrationof VOCs released from coating and furniture generally decreases intime whereas the release of certain substances depends on punctualhuman activities or even human breathing (Ekberg, 1994; Phillips,1997; Miekisch et al., 2004). Primary emissions of VOCs constitute themajor source in new or renovated dwellings during the first months,while physical and chemical deterioration of buildings material(named secondary emission) later becomes the main mechanisms ofVOC release (Wolkoff and Nielsen, 2001; Yu and Crump, 1998). IndoorVOC concentrations depend on the total space volume, the pollutantproduction and removal rates, the air exchange rate with the outsideatmosphere, and the outdoor VOC concentrations (Salthammer,1997).

Table 2Example of VOCs found in indoor air

Compound CASa Priority list Indoor concentration(µg m−3)

Recommendedvalues (µg m−3)

IARCc Health effects References

EPA OQAIb WHO Min Max Average

Acetaldehyde 75-07-0 × HP 3.24 119 18.9 LFCd 2B Respiratory disorders, irritation of theeyes

Weisel et al. (2005)78.0 12.0 Mosqueron and Nedellec

(2002)Benzene 71-43-2 × HP × 0.48 364 2.90 0.17 1 Immunological disorders, leukemia,

neurological effectWeisel et al. (2005)

141 1.57 LFCd Edwards et al. (2001)Dieldrin 60-57-1 × P 6.00×10−4 7.00×10−5 – 3 Neurological effect, cancer of the liver Mosqueron and

Nedellec (2002)Dichlorvos 62-73-7 × HP 2.24 0.455 – 2B Neurological effect, cancer of the liver Mosqueron and

Nedellec (2002)Formaldehyde 50-00-0 × HP × 11.2 53.8 20.1 1000 (1 h) 2A Respiratory disorders, irritation

of the eyesWeisel et al. (2005)

62.3 33.0 60e Edwards et al. (2001)Naphthalene 91-20-3 × × 2.20 90.1 – 2B Mosqueron and

Nedellec (2004)Tetrachlorethylene 127-18-4 × P × 0.10 20.9 0.56 250 2A Neurological effect, renal disorder Weisel et al. (2005)

73.6 1.38 LFCd Mosqueron andNedellec (2002)

Toluene 108-88-33

× P × 2.83 122 10.1 260 3 Neurological effect Weisel et al. (2005)

247 14.6 Edwards et al. (2001)Trichlorethylene 79-01-6 × P × 0.04 7.84 0.12 2.3 2A Neurological effect, cancer

of the testiclesWeisel et al. (2005)

41.8 0.86 LFCd Mosqueron andNedellec (2002)

a Chemical Abstract Service.b HP: High Priority, P: Priority.c International Agency for Research on Cancer (IARC) classification: Group 1: agent carcinogenic to humans, group 2A: agent probably carcinogenic to humans, group 2B: agent

possibly carcinogenic to humans, Group 3: agent not classifiable as to humans, group 4: agent not carcinogenic to humans, ns: non-study.d NIOSH recommended for the carcinogenic Lowest Feasible Concentration.e Canada (1987).

400 B. Guieysse et al. / Biotechnology Advances 26 (2008) 398–410

Typical air exchange rates in rooms without mechanical ventilationsystems range from 0.1 to 0.4 h−1. Indoor VOC concentrations aregenerally higher than outdoor concentrations because VOCs can bereleased from human activities and awide variety of materials such asfloorings, linoleum, carpets, paints, surface coatings, furniture etc (Yuand Crump, 1998). Salthammer (1997) for instance showed furniturecoatings could release 150 VOCs (mainly aliphatic and aromaticaldehydes, aromatic hydrocarbons, ketones, esters and glycols) atTotal VOC (TVOC) concentrations up to 1288 μg m−3 in test chamberstudies and TVOC emission rates as high as 22,280 µg m−2 h−1 havebeen recorded from vinyl/pvc flooring (Yu and Crump, 1998). Moldsand bacteria can contribute significantly to the presence of particles(spores) and VOCs in indoor pollution (Schleibinger et al., 2004).Microbial development in buildings can be found in places wherehumidity accumulates including defective heating and air condition-ing systems, garbage disposal, bathrooms, water leaks etc. and hasbeen shown to provoke toxic and allergenic responses. Thus, althoughthe individual concentrations of each contaminant are generally low(µg m−3), several hundreds contaminants can be found simulta-neously, resulting in significant TVOC levels. Kostiainen (1995) foundthat individual concentrations of selected pollutants were 5–1000times higher in 38 Finish sick-houses (defined as houses in whichpeople experienced symptoms associated with SBS) than their meanconcentrations in 50 normal houses used as reference, with over 200VOCs being simultaneously detected in 26 houses. The same studyalso reported a maximal TVOC concentration of 9538 μg m−3 in onesick house compared to the mean concentration of 121 μg m−3

recorded in the normal houses. Likewise, Brown and Crump (1996)recorded TVOC concentrations up to 11,401 μg m−3 in UK homes andDaisey et al. (1994) reported indoor TVOC concentrations of 230–700 µg m−3 (geometric mean of 510 µg m−3) in 12 Californian officebuildings. It is not easy to correlate the TVOC concentration withhealth effects because this generic parameter does not reflect theindividual differences in toxicities found among indoor air VOCs;however, experience of eye, nose or mouth irritation has beenreported at 5000–25,000 µg TVOC m−3 (Andersson et al., 1997).

Although indoor VOCs such as benzene or some polycyclicaromatic hydrocarbons are recognized as human carcinogens, a directassociation between exposure to VOCs and SBS symptoms or cancerhas not been fully established at typical indoor air concentrations(Wallace, 2001). Several studies however correlated exposure to lowconcentrations of these pollutants with increased risks of cancer oreye and airways irritations (Table 2) (Vaughan et al., 1986, Wallace,1991, Wolkoff and Nielsen, 2001). Symptoms such as headache,drowsiness, fatigue and confusion have been recorded in subjectsexposed to 22 VOCs at 25 μg m−3 (Hudnell et al., 1992). Likewise,exposure to 1000 μg m−3 of formaldehyde causes coughing and eyeirritation. In addition, many harmless VOCs can react with oxidantssuch as ozone, producing highly reactive compounds that can bemoreharmful than their precursors, some of which are sensory irritants(Sundell, 2004; Wolkoff et al., 1997; Wolkoff and Nielsen, 2001).Finally, most of the reported concentrations based on stationarymeasurement might lead to an underestimation of the real exposuredose of the subjects evaluated in epidemiological studies becauseconcentrations in the breathing zone could be 2 to 4 times higher thanthose recorded with traditional methods (Rodes et al., 1991; Wallace,1991; Wolkoff and Nielsen, 2001).

4. Indoor air treatment

Existing solutions to poor indoor air quality include combination ofactions such as removing the pollutant sources; increasing ventilationrates and improving air distribution; and cleaning the indoor air (USEPA, 2008). Although certain furniture or appliance-manufacturersare already phasing out the use of formaldehyde, removing thepollutant sources is only possiblewhen these are known and control istechnically or economically feasible, which is seldom the case. Newsubstances are constantly detected and classified as hazardous andmany sources can release compounds for years. In addition, thereare fears that many air pollutants are yet to be discovered (Otakeet al., 2001; Carlsson et al., 2000; Muir and Howard, 2006) and pre-ventive approaches might therefore be needed to ensure indoor air

Table 3Current and emerging indoor air treatment methods: principle, examples and limitations

Method Principle

Current methodsFiltration Air is passed through a fibrous material (often coated with

a viscous substance), which is efficient for particleremoval but not gases. Filters are compact and commonlyused but their efficiency decreases as they becomesaturated (fouling). Microorganisms can also develop infilters and particles reemission might occur.

Electrostatic precipitatorwith ionization

An electric field is generated to trap charged particles.Electrostatic precipitators are often combined with iongenerators that charge particles. Remove efficientlyparticles, are, compact, commonly used but can generatehazardous charged particles.

Adsorption Air pollutants are adsorbed onto activated carbon orzeolites, often as filtration post-treatment. The adsorbentmight be too specific and might saturate fast because thepollutant are not destructed. There is therefore a potentialrisk of pollutant reemission.

Ozonation Ozone is generated to oxidize pollutants. Only removesome fumes and certain gaseous pollutants and mightgenerate unhealthy ozone and degradation products.Ozone-based purifiers are not recommended by theAmerican Lung Association.

Photolysis High energy ultra violet radiation oxidizes air pollutantsand kills pathogens. It can however only remove somefumes and some gaseous pollutants and might releasetoxic photoproducts. Accidental exposure to UV light isharmful and UV irradiation is energy consuming.

Photocatalysis High energy ultra violet radiation is used in combinationwith a photocatalyst (TiO2) to generate highly reactivehydroxyl radicals that can oxidize most pollutants and killpathogens. This energy-intensive method is increasinglypopular and suitable for a broad range of organicpollutants.

Emerging methodsMembrane separation Pollutants are passed through a membrane into another

fluid by affinity separation. This method is normallyrecommended for highly loaded streams and has yet to beproven at low VOC levels. If the separated VOCs are notreused, membrane filtration must be completed with adestruction step.

Enzymatic oxidation Air pollutants are transferred into an aqueous phasewhere they are degraded by suitable enzymes Littleinformation is however available concerning theefficiency of the commercial system (Air and WaterSolution Inc., USA, http://www.srebiotech.com/) and newenzymes must be supplied periodically.

Botanical purification Air is passed though a planted soil or directly on theplants. The contaminants are then degraded bymicroorganisms and/or plants, the precise mechanismsbeing unclear. Although the efficiency of botanicalpurification has not been fully proven, a number ofdevices have been patented and several commercialproducts are available.US patent 6,676,091 for instance discloses a device whereair is forced directly through a vertical (or slightlyinclined) porous material serving as support forhydroponic plants, the plant's main purpose being tosupport the activity of pollutant degradingmicroorganisms in their rizosphere. This device is ratherlarge in regards to other technologies but can be use forinterior design purposes.

Biofilters andbiotrickling filter

Air is passed through a packed bed of a solid supportcolonized by attached microorganisms that biodegradethe VOCs. In one configuration, air was purified throughlava rocks covered with a geotextile cloth supportingmosses (Darlington et al., 2001).


contaminants are maintained below satisfactory levels at all times.Natural aeration is the easiest alternative but it is often not possiblebecause of outdoor weather, external pollution conditions (Ekberg,1994; Daisey et al., 1994) or issues of security, safety in high buildings,

climate control, or noise. Periodical air refreshing is often not efficientbecause many indoor air pollutants are constantly released. Hence,forced ventilation is still one the most common methods used for airtreatment (Wargocki et al., 2002). The improvement of indoor airquality and energy savings are encouraged in the EU and bymovements such as the “Green Building” (US Green Building Council,2008), which means that forced ventilation should be reduced at thesame time as IAQ should be improved. In a worst case scenario (noheat recovery from ventilation) the energy requirements for heating acommercial office of 100 m2 and 2.5 m of height at 23 °C ventilatedwith outdoor air (4 °C) at an air exchange rate of 3 room volumes perhour is approx. 3420 kWh/month (or about 340 USD, based onresidential electricity prices in 2006). Consequently, there are fewalternatives left than purifying the air inside the building.

Current methods for air purification include combinations of airfiltration, ionization, activated carbon adsorption, ozonation, andphotocatalysis (Table 3). These processes can be integrated into thecentral ventilation system (in duct) or used in portable air purifiers (orair cleaner) designed for limited spaces. Efficient strategies for particleremoval are now well established and include combinations offiltration and electrostatic precipitation. The situation is howeververy different for VOC removal. For instance, in a study conducted tocompare several commercial air purifiers, Shaugnessy et al. (1994)concluded that, although high efficiency particles air filters (HEPAfilters) and electrostatic precipitators were highly efficient for particleremoval, none of the techniques tested (HEPA filtration, electrostaticprecipitation, ionization, ozonation, activated carbon adsorption)could significantly remove formaldehyde. A similar studywas recentlyconducted to compare 15 air cleaners with a mixture of 16representative VOCs (Chen et al., 2005). The technologies evaluatedincluded sorption filtration, ultraviolet-photocatalytic oxidation (UV-PCO), ozone oxidation, air ionization and a botanical purifierprototype (where contaminated air was blown through the rizosphereof plants and contaminants were in principle removed by soilmicroorganisms, the plants or their enzymes through variousmechanisms). The results were:

1. Among the 7 air cleaners using activated carbon in combinationwith HEPA filter and/or ionizer, the best single pass removalefficiencies achieved for formaldehyde, toluene and dodecanewere4, 32 and 39%, respectively (these contaminants were selectedfor being representative of soluble, semi-soluble and non-polarsubstances).

2. The commercial UV-PCO purifier did not effectively remove any ofthe tested VOCs, although a “properly designed” device waseffective for certain VOCs (no data presented).

3. None of the ozone generator systems significantly removed any ofthe VOCs tested; some even released ozone to concentrationsmuchhigher than the safety limit set by the OSHA (Occupational Safetyand Health Administration).

4. The botanical purifier was able to remove around 20% offormaldehyde (single pass) but no toluene and only 4% ofdodecane.

From this data, the authors concluded that only the biologicalsystem significantly removed very volatile organic compounds, suchas formaldehyde, in contrast to the adsorption processes thatgenerally only satisfactorily removed the poorly soluble contaminants.The overall elimination capacity (g VOC removed d−1) of the botanicalpurifier for formaldehyde was however lower than the best activatedcarbon based device because the biological purifier could not beoperated at high air flows. An air exchange rate of 3 room volumes perhour is generally recommended for indoor air treatment, whichmeans that very large amounts of air must be treated into relativelysmall units (for suitable use in the indoor spacewithout visual or noisenuisances). This might be difficult to achieve in botanical purifierswhere air is ventilated into the soil through the roots.

http://www.srebiotech.com/

Table 4Biodegradability of typical indoor VOCs

Substance Biodegradabilitya Henry's law constants Biological treatment

Hb (atm m3

mol−1)References Inlet concentrationc

(mg m−3)Removalefficiency (%)

Biologicaltreatmentd

References

Acetaldehyde(Ethanal; CH3CHO)

3 5.88 10−5 Zhou and Mopper(1990)

18.1–180.1e 40–80 B Mohd Adly et al. (2001)

7.69 10−5 Sander (1999)5.88 10−5 US EPA (1982)

Benzene (C6H6) 2 6.25 10−3 Staudinger and Roberts(1996)

1.6e 9–77 B Ergas et al. (1992)

5.55 10−3 US EPA (1982) 0.32–1.28e 50 to 60 BF Wolverton et al. (1989)4.76 10−3 Sander (1999) 0.048–0.48e 20 PW Darlington (2004)

Formaldehyde (Methanal;HCHO)

3 3.33 10−7 Sander (1999) 0.12–0.49e 50 to 60 BF Wolverton et al. (1989)

3.23 10−7 Zhou and Mopper(1990)

0.018–0.18e 90 BF Darlington (2004)

3.13 10−7 Staudinger and Roberts(1996)

Naphthalene (C10H8) 1 4.76 10−4 Sander (1999) 0.494e 75 TPPB MacLeod and Daugulis(2003)

4.76 10−4 US EPA (1982)Tetrachlorethylene(Tetrachloroethene; C2Cl4)

1 2.78 10−2 US EPA (1982) 0.678e 12–49 B Ergas et al. (1992)


0.36–4.80e 0–8 BTr Torres et al. (1996)

1.56 10−2 Sander (1999)Toluene (Methylbenzene;C6H5CH3)

2 6.67 10−3 US EPA (1982) 1.88e 14–78 B Ergas et al. (1992)


753.5 50 MS Ergas et al. (1999)

0.226–0.301e BF Darlington et al. (2001)0.057–0.57e BF Darlington (2004)

Trichlorethylene(Trichloroethene; C2HCl3)

1 9.09 10−3 Sander (1999) 107.44 30 MS Parvatiyar et al. (1996)

1.12 10−2 US EPA (1982) 0.081–0.81e 0 BF Darlington (2004)1.00 10−2 Staudinger and Roberts

(1996)0.054–2.149e 50 to 60 BF Wolverton et al. (1989)

0.01–0.04e 0–24 BTr Torres et al. (1996)

a 1= low biodegradability, 2=moderate biodegradability, 3=good biodegradability (Shareefdeen and Singh, 2005; Devinny et al., 1999).b Under standard conditions.c Concentrations close to the average concentration observed in indoor air.d B = Biofiltration; MS = Membrane Separation; BF = Botanical Filter; TPPB = Two-Phase Partitioning Bioreactor; BTr = Biotrickling Filter.e In mixture with other compounds.


Membrane mediated VOC removal in indoors air has been alsorecently evaluated. Formaldehyde and benzene were significantlyremoved at concentrations of 11,500 and 6500 µg m−3, respectivelyusing a zeolite membrane operated at permeation fluxes of 0.01 and0.0056 gm−2 h−1, respectively (Aguado et al., 2004). Further studiesmusthowever demonstrate pollutant removal at concentrations relevant toindoors environments (approx. one order ofmagnitude lower than thosetested) and provide solutions to destroy the VOCs following separation.

5. Biological treatment of indoor air

There is little data available on the biological removal of VOCs fromindoor air and all the studies hitherto conducted have, to the best ofour knowledge, focused on botanical purifiers. In a pioneer studysupported by the NASA, Wolverton and co-authors demonstrated thepotential of plants (and their rizosphere) to remove indoor VOCs insealed chamber. In their earliest study (Wolverton et al., 1984), theauthors found that several plants could remove formaldehyde at19,000–46,000 µg m−3 to levels lower than 2500 µg m−3 (detectionlimit) in 24 h. Similar studies were conducted with benzene andtrichloroethylene atmore relevant concentrations of 325–2190 µgm−3

(Wolverton et al., 1989). It was then found that the 8 plants testedcould remove benzene by 47–90% in 24 h compared to 5–10% in thecontrol tests, and that the rizosphere zone was the most effective areafor removal. Orwell et al. (2004) later investigated the potential ofindoor plants for removing benzene in sealed chamber (0.216 m3) and

found that microorganisms of the plant rizosphere were mainlyresponsible for benzene removal (40–80 mg m−3 d−1). These resultswere obtained at high initial benzene concentrations (81,000–163,000 µg m−3) and benzene removal rate increased linearly withthe dose concentration, suggesting the system might be inefficientunder typical indoor air conditions. However, the same team morerecently demonstrated that plants significantly reduced toluene andxylene at indoor air concentrations of 768–887 µg m−3 (Orwell et al.,2006) and even the TVOC concentration in office buildings duringfield testing under real conditions (Wood et al., 2006). Unfortunately,the divergences in toluene removal reported in the studies of Chenet al. (2005) and Orwell et al. (2006) cannot be explained, especially asthe prototype used in the earlier study was not fully described. Manyparameters such as the interfacial areas, the moisture content, and thetype (hydrophobicity) of the biomass used can influence pollutantremoval in biological purifiers. There is therefore a need for a morecoordinated research in the area. Various botanical purifiers have alsobeen patented (i.e. US5407470, US5277877) but such devices have notreached a broad market and no data on pollutant removal underrelevant conditions is available. Research on the development of acommercial biological purifier has been carried out at the University ofGuelph, Canada (Darlington et al., 2000; Air Quality Solution Ltd). Inone configuration, air was purified through lava rocks covered with ageotextile cloth supporting mosses (Darlington et al., 2001). Thisdevice was operated at relevant influent levels equal or lower than300 µgm−3 and displayed a purification efficiency of 30% at the lowest


air flow treated. Water was also added to the filter to compensate forwater losses through evaporation (approx. 20 L d−1 in 120 m2 and640 m3 room). In a second configuration, disclosed in US patent6,676,091 from the same author, air is forced directly through avertical (or slightly inclined) porous material serving as support forhydroponic plants which its main purpose is to support the activity ofpollutant degrading microorganisms in the rizosphere.

From the studies herein presented, it appears that the role of plants inbotanical purifier is often suspected to support amicrobial activity that isresponsible for pollutant removal. Direct pollutant accumulation ordegradation by plants is however known to occur during phytoremedia-tion of contaminated soils (Newman and Reynolds, 2004) and the abilityof plant leaves to directly take up and remove pollutants during airtreatment is still debated (Wolverton et al., 1984; Schmitz et al., 2000;Schäffner et al., 2002). A recent studyhas suggested that bacteria growingon plant leaves could also contribute to VOC biodegradation (Sandhu etal., 2007). More generally, there is growing evidence of the complexity,and importance of interactions between plants and bacteria (Dudler andEberl, 2006) and research in this area is of utmost importance for IAQ.There is a lack of peer-reviewed data available in the literature and anurgent need to improve our understanding of the fundamental mechan-isms of VOC uptake or release by plants and their microbial hosts(Kesselmeier and Staudt, 1999). The following discussion will thereforefocus on the more established microbial degradation mechanisms.

5.1. Influence of type of VOCs

The biological treatment of organic compounds is based upon thecapability of microorganisms to use these molecules as sources ofcarbon, nutrients and/or energy or to degrade them cometabolicallyusing unspecific enzymes. The intrinsic biodegradability of an organiccompound depends on many factors such as its hydrophobicity to themicrobial population, the most soluble being generally the mostbiodegradable, or its toxicity. Toxicity effects, which sometimes limitthe biological treatment of industrial air, are likely not a problem at theconcentrations found in indoor air and this will not be discussed furtherin this review. Many VOCs are rather small molecules that aremoderately soluble and in fact, are biodegradable (Table 4) althoughcertain xenobiotic compounds, such as chlorinated compounds (i.e.tetrachloroethylene), may be recalcitrant. Synergetic or negativeinteracting effects within pollutant mixture should also be taken intoconsideration (Yu et al., 2001). Given the high number of VOCssimultaneous found in indoor air, and the huge variations in structuresand properties, a biological process suitable for indoor air treatmentshould rely on diverse, versatile and adaptive microbial communities toensure all pollutants are removed. This can be achieved in fixed biofilm-based reactors where high microbial diversity and cell proximity favourcellular exchanges (Molin and Tolker-Nielsen, 2003; Singh et al., 2006),acclimation (long cell residence time) and synergetic effects undervarious growth conditions by the establishment of substrate concentra-tion gradients through the biofilm (Beveridge et al., 1997; Marshall,1994). Completing or combiningbiodegradationwith a physicochemicalpost-treatment is also possible to ensure the complete removal of allpollutants. Finally, great variations in total and individual pollutantconcentrations leading, for instance, to long periods of time when agiven compound is not found in the indoor air could lead to permanentor momentary losses in catabolic ability. Such effects need to be furtherstudied and possibly prevented as discussed below.

5.2. Influence of low concentrations on biomass productivity and transferrates

During the biodegradation process, the concentration of an organicpollutant in the micro-environment where the microorganisms arefound has a profound impact onmicrobial activity and ultimately on thepollutant removal rate. At reasonably high substrate concentrations, the

organic pollutant can be metabolized and used to synthesize morebiomass in a process that self-regenerates the biocatalyst. When theconcentration is decreased further, a critical level is reached belowwhich new cells are no longer produced. Nevertheless, the contaminantcan still be biodegraded if a significant active biomass is available and ifthe gene(s) responsible for the production of the enzyme(s) required forthe pollutant degradation is (are) still expressed (Kovárová-Kovar andEgli, 1998). Gene expression depends, among other factors, on theconcentration of the enzyme substrate which can be the pollutant orother molecules. In this context, it is crucial to compare the lowconcentrations at which indoor VOCs are typically found with knownthreshold for microbial growth and biodegradation.

Toluene indoor air concentrations of 0.58–17 μg m−3 have beenreported in Californian office buildings (Daisey et al., 1994). Assumingtoluene must first transfer into an aqueous phase before beingbiodegraded, the maximum aqueous toluene concentration (Caq⁎) atwhich microorganisms will be exposed to can be calculated from theHenry's law constant (H) coefficient:

C4aq ¼ Pi

Hið1Þ

Where Pi is the partial pressure of the target contaminant in thegas phase and Hi its Henry constant. For toluene (H=6.67 10−3 atmm3

mol−1; Table 4), this will result in a Caq⁎ of 2–60 ng L−1 under normalconditions of temperature and pressure. If toluene is removed by 90%,microorganisms would actually be exposed to concentrations of 0.2–6 ng L−1 (under continuous treatment at a steady state). At suchconcentration, toluene can be reasonably considered as the limitingsubstrate if it is the only carbon source available. By comparison, thethreshold growth concentration of bacteria from drinking-waterbiofilm has been estimated to about 0.1 μg L−1 (van der Kooij et al.,1995) which is in the same range of reported toluenemineralization ataqueous concentrations of 0.9 μg L−1 with active bacteria (Roch andAlexander, 1997). Hence, from the data currently available, it seemsunlikely that indoor air VOCs can support growth. However, micro-organisms might still be capable to use certain VOCs as energy sourceformaintenance or cometabolically biodegrade them by using anothersubstrate for growth.

The specific biomass growth rate (µ, h−1) at sub-inhibitorysubstrate concentration can often be expressed as:

μ ¼ μmaxSKs þ S

−kd ð2Þ

Where µmax (h−1) is the maximum specific growth rate, Ks is thesaturation constant (mg L−1), kd is the endogenous decay coefficient(h−1) and S is the limiting substrate concentration (mg L−1). Thesubstrate uptake volumetric rate is equivalent to:

rsu ¼ μXY

þmX ð3Þ

where X is the biomass concentration, Y is the true biomass yield (gbiomass g substrate −1), and m is the maintenance coefficient. Thisequation shows that substrate consumption for maintenance canoccur even under no growth conditions (μmaxS / (Ks+S)bkd), althoughcell decay would cause a continuous decrease in catabolic activity.

Based on the toluene biodegradation kinetics reported byAlagappan and Cowan (2003) in Pseudomonas putida F1 cultures(µmax=0.37 h−1 and Ks=0.44 mg L−1), the specific cell production rateunder indoor conditions should range from 5×10−5–1.7×10−6 h−1.These values are far below the cellular maintenance rates reported inthe literature for aromatic compounds (0.019 h−1 for ethylbenzeneand 0.016 h−1 for benzene), and the death cells coefficients for P.putida F1 during the degradation of toluene (0.06 h−1; Alagappan andCowan, 2003). Thus, in this particular situation, neither wouldpollutant supply meet maintenance requirements nor would thespecific growth rate meet the cellular decay rate.


Although these kinetic parameters were obtained at pollutantconcentrations higher than those found in indoor air, this simplecalculation shows that indoor air biological treatment will likelyrequire the development of specific methods to provide and maintaina suitable catabolic activity. First, due to the complexity andvariability of indoor air, an inoculum that possesses the suitablecatabolic ability might be difficult to obtain. These microorganismswould also likely need to be pre-cultivated at higher VOC concentra-tion to obtain a significant cell number in a relative short time, whichmight impair their ability to take up substrates at trace levels(microorganisms can loose selective traits when the correspondingselection pressure is released). Attached growth should therefore berecommended for inoculum preparation to combine, through theestablishment of substrate concentration gradients inside the biofilm,microbial growth at the liquid-biofilm interface with continuousselection of microorganisms acclimated to low-substrate concentra-tion inside the biofilm (Beveridge et al., 1997). Second, maintainingcatabolic activity (and not only cell mass or cellular activity) could bechallenging as microorganisms can loose their ability to biodegradecertain substrates when deprived from them during long periods oftime. Biofilm systems could ensure constant performance underfluctuating operating conditions by allowing substrate accumulationat the biofilm interface until degradation becomes possible (Singhet al., 2006). As mentioned above, attached microorganisms also formdiverse and dynamic communities (cell communication and cellularexchanges such as horizontal gene transfer being favoured by the cellproximity) that can respond more quickly to changes, which in returnfavours functional redundancy (Rittmann and McCarty, 2001).Biofilms are in fact ubiquitous and prevail under diluted environment(Beveridge et al., 1997). Finally, even under conditions when suitabledegradation-enzymes are expressed, microbial activity must becapable to reduce the contaminant at concentration low enough topermit significant mass transfer. Roch and Alexander (1997) showedtoluene mineralization at 0.9 μg L−1 but the pollutant still remained at79 ng L−1 after 8 days of incubation. Similar findings were reported byPahm and Alexander (1993) when studying the biodegradation of p-nitrophenol at trace concentration although addition of a secondarycarbon source was capable to trigger pollutant removal at concentra-tions of 1 μg L−1. However, the feasibility of removing estrogens at100 ng L−1 to below 2.58 ng L−1 (detection limit) with pure laccase

Fig. 1. Schematic presentation of a simple steady stat

from T. versicolor was recently demonstrated (Auriol et al., 2007),showing biological systems should be able to perform at indoor airconcentrations.

Clearly, the development of biological methods for indoor airfiltration faces several challenges and requires more research on themicrobial mechanisms of acclimation, survival, substrate recognition,accumulation and uptake at trace concentration. Most publishedbiodegradation studies have been conducted under irrelevant condi-tions (single substrate, high substrate concentration, suspendedgrowth) using isolates cultivated in the laboratory. Current kineticsdata might therefore underestimate the capacity of natural strains totake up trace pollutants (Subba-Rao et al., 1982; Pahm and Alexander,1993). Low concentrations are common in the environment andcertain microorganisms have developed original survival strategiesunder such conditions by for instance accumulating limiting substratebefore starting to growth (Singh et al., 2006). Newmodels to correlategrowth with substrate concentration are therefore needed at traceconcentration, as suggested by Butterfield et al. (2002) in a study ondrinking-water biofilm formation under carbon-limited conditions(b2 mg L−1).

The simultaneous presence of many contaminants in indoor airmight sustain microbial growth or, at least, induce pollutant miner-alization, as suggested by the experience of Pahm and Alexander(1993) described above. Furthermore, under starving conditions,certain microorganisms are capable to quickly increase their affinityfor the limiting substrates or acclimate to simultaneously mineralize ahigh number of carbon sources (Kovárová-Kovar and Egli, 1998, Troset al., 1996). Particles and other air macropollutants (carbon dioxide,carbon monoxide, ammonia, etc) or plants might also provide enoughenergy and carbon substrates for growth, as evidenced by themicrobial colonization of indoor surfaces and air handling systems orthe microbial activity recorded in the rizosphere of botanical purifier.In addition, certain microorganisms are able to grow both hetero-trophically and autotrophically (Larimer et al., 2003) or on myriads ofdifferent organic compounds (Chain et al., 2006). Such metabolicversatility would give obvious advantages under conditions wherenumerous potential carbon and energy sources are simultaneouslyfound at very low concentrations and would greatly enhance thetreatment of indoor air. The question is therefore not if microbialgrowth would occur, but if it will cause VOC reduction. For instance,

e mass balance analysis of indoor air treatment.


from field studies on the efficiency of plants to purify air from officebuildings, Wood et al. (2006) suggested that a TVOC concentration of100 ppb was sufficient to induce a biological response that couldreduce the TVOC concentration up to 75%. In addition, duringlaboratory studies on VOC removal by potted plants in sealed chamber,Orwell et al. (2006) observed the presence of toluene accelerated theremoval of x-xylene, although the reciprocal was not true.

Several authors have also challenged themass transfer andmicrobialuptake theories use to predict the effect of substrate concentration inbiological purifiers. Active transfer by enzymatic transformation has forinstance been reported and mechanisms of direct uptake at the air–cellinterface have been suggested. For instance, Miller and Allen (2005)reported that direct pollutant diffusion through the aqueous layersurrounding the biofilm could not explain the surprisingly highperformances of biological systems treating the highly hydrophobicalpha-pinene. Likewise, it has been suggested that the aerial mycelia offungi, which are in direct contactwith the gas phase,might promote thedirect uptake of VOC from the gas phase. This uptake is faster than if aflat biofilm of bacteria directly contacts the gas phase because of a highgas–mycelium interfacial area of the fungal mat and the highlyhydrophobic nature of the fungal cell wall (Arriaga and Revah, 2005;Kennes and Veiga, 2004; Van Groenestijn and Kraakman, 2005, Vergaraet al., 2006).

5.3. Impact of purification efficiency on design

A simple steady state mass balance analysis can be used to design adevice for indoor air purification (Fig. 1) based on the followingequation:

VdCdt

¼ QCin−QCout þ Rp ð4Þ

where V is the room volume (m3); C the TVOC concentration in theroom (μg m−3); Q is the refreshment flow rate (m3 h−1) through theroom,Cin is the TVOCconcentrationof the air entering the room(μgm−3),Cout is the TVOC concentration of the air leaving the room (μg m−3), andthe Rp is the TVOC production rate inside the room (μg h−1).

At steady state under completely mixed conditions within thecontrol volume analyzed dC/dt=0 and C=Cout. Assuming that Cin isnegligible compared to Cout (indoor air concentrations are usuallyhigher than outdoor), Eq. (5) gives:

Rp ¼ QC0 ð5Þ

where C0=Cout represents the initial pollutant concentration in theroom, before air is being cleaned. When an air treatment unit (i.e. abiological purifier) is started in the room the mass balance analysisbecomes:

VdCdt

¼ QCin−QCout þ Rp−Rb ð6Þ

Where, for a biological purifier, Rb is the TVOC biological removalrate (μg h−1).

If we assume that Rp is constant (which is unlikely as the rate ofVOC evaporation from coatings depends on the concentration of theVOC in the room and the concentration in the material) and Cin≈0, therate of TVOC biologically removed under steady state conditions canbe expressed as:

Rb ¼ Q C0−Cð Þ ð7Þ

If we now consider the biological purifier, Rb can also be expressedas:

Rb ¼ Qb Cbin−Cbout

� � ð8Þ

Where Cbinand Cbout

are the TVOC concentration in the air enteringand leaving the biological purifier, respectively; and Qb is the air flowtreated by the biological filter.

By definition, the single pass efficiency (%) of the biological purifieris expressed as:

η ¼ Cbin−Cbout

Cbinð9Þ

Based on the assumption of completely mixed conditions in theroom Cbin

=C, then:

Rb ¼ ηCQb ð10Þ

Combining Eqs. (8) and (11) gives the following expression for Qb:

Qb ¼ C0−CηC

Q ð11Þ

The required biological purifier air flow is therefore a function ofthe single pass biofiltration efficiency, the initial and required TVOCconcentrations, and the refreshment rate. It can also be expressed as afunction of the ratio between initial and final concentrations in theroom (A=C0/C), which is another expression of the removal efficiencyaccording to:

Qb ¼ A−1η

Q ð12Þ

The overall performance of the purification device can then beexpressed as its effectiveness (R, %) according to:

R ¼ C0−CC0

ð13Þ

Based on this equation, a simulation was made to determine Qb

under varying R and η values at C0=500 μg m−3 (Fig. 2) as typicalTVOC values in sick houses range from 100 to 1000 μg m−3. Therefreshment rate was assumed to 0.35 h−1 in a room of 100 m3

(Q=35 m3 h−1) as described by Wolkoff et al. (1991). This simulationshows that a purification flow (Qb) of approx. 400m3 h−1 is required toachieve a satisfactory overall purification effectiveness of 90% withη=80%. This is rather logical since the removal efficiency of anysystem is intrinsically limited by the low concentration of thepollutants. These values are in accordance with the recommendedair cleaner true effectiveness of 80% for particle removal, which isequivalent to providing 4–5 room volumes of clean air per hour(Shaughnessy and Sextro, 2006). Hence, it is not only the single passpurification efficiency of the biofiltration device but the overallpurification capacity (ηQb) that is important, explaining why theconcept of Clean Air Delivery Rate (CADR=ηQb, the amount of purifiedair delivered per unit or time) was introduced to evaluate andcompare the various devices proposed for air removal (Shaughnessyand Sextro, 2006). Interestingly, at equivalent CADR, purificationdevices with high single pass efficiencies should be preferred becauseof their lower energy requirement (lower required flow rate).

The biological purifier can be designed using another engineeringapproach based on the gas residence time (tres) in the purifier and thepurifier refreshment capacity (α):

tres ¼ Vb

Qbð14Þ

where Vb is the volume of the biological purifier.

α ¼ η � Qb

V¼ CADR

Vð15Þ

Fig. 2. Simulated performance of the effect of the biological purifier single pass efficiency (η, %) on the required biofiltration volumetric flow rate (Qb, m3 h−1) to achieve an overalleffectiveness R=80–95% (with R=(C0−C) /C0 where C and C0=500 µg m−3 are the steady state TVOC concentrations before and during the biological purifier operation, respectively)in a 100 m3 room with a refreshment rate of 0.35 h−1. The air cleaner effectiveness can also be expressed as the ratio A=C/C0.


Thus, the biological purifier volume required can be expressed as:

Vb ¼ α � tres � Vη

ð16Þ

Assuming a typical gas residence time (industrial applications) of30 s, a purifier refreshment capacity of 4 h−1, and a 80% single passefficiency, the biological purifier volume required for a 100 m3 roomwould be 4.2m3, which is a prohibitive volume in indoor environment.Even at such a short residence time as 2 s, the volume of the bioreactorneeded in our simulationwould be 278 L. By comparison, the biologicalpurifier developed by Darlington et al. (2001) was tested at surfaceloading rates of 90–720 m3 m−2 h−1, which is within the range ofoperation of industrial biotrickling filter and bioscrubbers (100–1000 m3 m−2 h−1; Van Groenestijn and Hesselink, 1993). The 1920 L(9.6 m2) bioscrubber (not including plants and aquarium used forwater circulation) was also operated in a 640 m3 room at influenttoluene, ethylbenzene and xylene concentrations of 0–300 µg m−3.Single pass efficiencies of approx. 10–30% were achieved dependingon the flow (the higher the flow, the lower the single pass efficiency)and temperature, which accounted for a CADR of 720 m3 h−1 equi-valent to a refreshment rate of 1.1 h−1. Although it is difficult to drawconclusions from this single study, the data provided seems con-sistent with our simulation. It clearly shows that the engineering ofcompact biological purifiers with high effectiveness will be an im-portant challenge.

The model described above can also be used to simulate thedynamic changes in pollutant concentration in a room where airpurification is started by using Eq. (4) expressed as:

VdCdt

¼ QC0−QC−ηQbC ð17Þ

Which can then be integrated as:

Ct ¼ C0 exp −Btð Þ þ AB

1− exp −Btð Þð Þ ð18Þ

With

B ¼ Q þ ηQb

V¼ Q þ CADR

Vð19Þ

And

A ¼ QC0

Vð20Þ

In Eq. (18), C0exp(−Bt) represents the combined VOC removal fromrefreshment and biofiltration and A

B 1− exp −Btð Þð Þ represents theproduction within the room. Using the example described above(V=100 m3, Q=35 m3 h−1, C0=500 µg m−3), Fig. 3 shows that at therange of purification flows required for such application, steady statesconditions are achieved rather quickly (1–2 h). Similar models areused to estimate the single pass efficiency of purification devices insealed chamber test where pollutant are introduced at a certainamount but where there is no production (Chen et al., 2005). Thus,Wolverton et al. (1989) reported a decreased in benzene concentrationfrom 765 to 78 µg m−3 in 24 h in a sealed chamber containing a plant,which resulted in a B coefficient in Eq. (19) of 0.1 h−1. The B coefficientis composed of the pollutant leakage rate from the system (Q/V) andthe pollutant removal in the air purifier (CADR/V=purifier refresh-ment capacity). The same author conducted a leak experiment whichallowed calculating the leak contribution to approx. 0.01 h−1. Hence,the botanical purifier used in this study generated an amount ofpurified air equivalent to 0.09 room volume per hour (CADR of0.075 m3 h−1) and would not significantly improve IAQ under realisticconditions. Low refreshment rates of 0.02–0.3 h−1 were also achievedby Orwell et al. (2006) in sealed-chambers containing potted plantsand initially supplied with 768–886 µg m−3 of m-xylene or toluene,based on VOC exponential removal rate constants of 0.52–7.44 d−1.Likewise, Chen et al. (2005) achieved the highest CADR of 8.3 m3 h−1

(refreshment rate of 0.15 h−1) with the botanical purifier compared to

Fig. 3. Simulated changes in TVOC concentration (C, µgm−3) in a 100m3 roomwhere airpurification is started at t=0 at a flow of 500 (dashed line), 300 (plain line) or 100(circles) m3 h−1 in a biological purifier with 90% single pass efficiency. The refreshmentrate in the room is 0.35 h−1 and the initial TVOC concentration is 500 µg m−3.


values above 200 m3 h−1 with other portable devices. Despite this, asignificant TVOC removal was recorded when using potted plantsduring field testing in office (Wood et al., 2006) and even if suchresults should be reproduced under better controlled conditions, theymight indicate that our current evaluation models are inadequate.

5.4. Humidification and biohazards

Since biological purifiers are typically saturated with water andsince indoor air treatment requires high flows, indoor biologicalpurification might increase the moisture content in the room orbuilding where it is used. This beneficial effect when indoor air is toodry (moisture contents of 30–60% are generally recommended forcomfort) could also trigger to the excessive growth of fungi withnegative impact on IAQ (Schleibinger et al., 2004), although theseeffects are still uncertain (Robbins et al., 2000; Pasanen, 2001).Darlington et al. (2000) for instance reported that the use of an indoorbiological purifier significantly increased the concentrations of totalsuspended spores, although these values were similar to concentra-tions found in flats containing house plants, and still remained withinhealthy levels (100–200 CFU m−3). In addition, none of the 17 fungalspecies identified was known to be pathogenic. Likewise, Ottengrafand Konings (1991) reported that the concentration of microbialgerms (mainly bacteria) in the outlet of full scale industrial biofilterswas within the range of typical indoor air concentration, and onlyslightly higher than typical outdoor air concentrations, which wasmore recently confirmed by Zilli et al. (2005). There is however toolittle data available and the potential release of microorganisms fromindoor biological purifiers (especially in the case of faulty equipmentor accidents) should be better studied and prevented if necessary.

5.5. Esthetic, noise, purification perception

Besides being efficient for pollutant removal, indoor air purifiersmust be esthetic (unless integrated into the ventilation system) andsilent, which is rather challenging considering theflow required and thevolumetric constrains. As mentioned above, indoor air pollutioninvolves many types of pollutants (particulates, inorganic etc), whichconcentrations must all be reduced below a certain “perception” levelfor users to feel the improvement of air quality. Especially, the effects ofmacropollutants such as CO2 and H2O should be considered.

5.6. Evaluating performance

Setting-up realistic purification goals is difficult as there are onlyfew guidelines on indoor VOC concentration. Evaluating performance

is no less challenging due to the analytical difficulty to detectnumerous substances at very low concentrations, the lack of knowl-edge about contaminants and their effects, and because of the numberof associated effects of air purification (i.e. moisture, temperature,physiological impacts on user). Ultimately, blind testing in sick-buildings that could correlate pollutant removal with customer/usersatisfaction (by survey, measurement of productivity, etc) and healthimprovements would be necessary. Unfortunately, such methodscannot be used during phases of design and optimization foreconomical reasons and because of the need for well-defined andreproducible testing conditions, replicate and control. Hence, theefficiency of air purifiers is generally evaluated by either directmeasuring of the single pass efficiency or by using test-chambers. Inthe first case, an artificially contaminated air stream is passed at acertain flow through the purification device and the concentrations ofthe target contaminants are measured at the inlet and outlet of thepurifier (Howard-Reed et al., 2002). This method is rather simple andthe influence of parameters such as the effluent composition or thetreatment flow can easily be tested under continuous inlet aircomposition. Test-chamber assays are however often preferred,perhaps because they offer more flexibility and better simulate theindoor environment. Here, pollutants are injected in a hermeticalchamber equipped with the air purifier as well as various sampling,analysis, and air conditioning devices and their concentrations aremeasured over time. The purifier efficiency can then be evaluatedusing the basic model described above. Depending on the size andcomplexity of the chamber, the pollutant can be replaced by polluting-material and treatment efficiency can be evaluated, for instance, interms of work productivity by monitoring the activity of human test-workers operating inside the chamber (Wargocki et al., 1999). The useof artificially contaminated air also brings its own challenges as it isvery difficult to determine a universal “model indoor air” due to thediversity and variability of pollutants concerned (Ekberg,1994; Yu andCrump, 1998; Otson et al., 1994). However, recent progresses havebeen made in that direction (Ondarts et al., 2007).

VOC analysis from indoor air normally requires large air samplesthat are passed through a solid or liquid absorbent that serves toconcentrate the contaminants, followed by further extraction/separa-tion and analysis (Crump, 2001). However, many pollutants are notknown or cannot be detected at indoor air concentration levels(Ondarts et al., 2007) and the costs of monitoring all knowncompounds would be prohibitive. In such cases, the TVOC (for detailson protocols see Crump, 2001) can be measured although thisparameter can be exclusive and does not take into consideration thedifferent intrinsic toxicities of each compound and interaction effectsbetween the pollutants (Wolkoff, 2003; Wolkoff and Nielsen, 2001;Molhave, 2003).

6. Designing biological purifiers

Common biological processes for VOC abatement include bio-scrubbers, biotrickling filter, and biofilters (Iranpour et al., 2005;Burgess et al., 2001; Delhoménie and Heitz, 2005; Revah and Morgan-Sagastume, 2005). In bioscrubbers, the air is washed with an aqueousphase into which the pollutants transfer, and the aqueous phase istransferred into a bioreactor where the pollutants are biodegraded. Inbiotrickling filters, microorganisms are grown on an inert material(plastics resins, ceramics etc). An aqueous solution containing thenutrients required for microbial growth is continuously distributedand recirculated at the top of the reactor and percolates by gravity,thus covering the biofilm with an aqueous layer. Contaminated air isintroduced as co- or counter current and the contaminants diffuse intothe aqueous phase where they are biodegraded. The purpose of thepackingmaterial is to facilitate the gas and liquidflows and enhance gas/liquid contact, to offer a surface for microbial growth, and to resistcrushing and compaction. In biofilters, air is passed through a moist


porousmaterialwhich supportsmicrobial growth.Water remainswithinthe packing material and is added intermittently to maintain humidityand microbial viability. The packing material is generally a naturalmaterial (peat, compost,wood shavings, etc,)which is biodegradable andprovides nutrients to the microorganisms although intensive researchhas been done to use synthetic materials (Jin et al., 2006).

Because they provide large interfacial areas for exchange, biofiltersare typically recommended for low-substrate concentrations andpoorly soluble substances. On the other hand, biotrickling filters andbioscrubbers allow higher surface volumetric loading rates and aremore suitable for conditions of fast transfer (high pollutant concen-tration) or when pH needs to be controlled. As indoor air biofiltrationimplies low transfer and high volumetric flow, it is not clear which ofthe configurations described above will be the best. However, biofilm-based technologies offer many advantages in regards of the microbialproperties required and the utilization of a biodegradable (i.e.compost) or bioactive (i.e. soil, plants roots, plant leaves) support forgrowth might allow microbial activity at low-substrate concentration.It is therefore not surprising that botanical purifiers, which are basedon configurations taking advantage of air exchange through the plantroot–soil or plant–foliage areas, have been more extensively studiedfor indoor air purification so far.

Emerging technologies could provide a more suitable platform forindoor air biological purification. Biological indoor air treatment canpotentially release dust, microorganisms, and water. These problemscan be simultaneously solved by using membrane bioreactors whichphysically disconnect the sorption step (air–water exchange) from thebiodegradation step. The use of membrane bioreactors for VOCremoval is nowadays only established at high pollutant loads (Ergaset al., 1999) but recent studies have demonstrated that such systemscould be efficient at indoor air concentration levels (Llewellyn andDixon, 2006, Ramis et al., 2007). The use of membrane could allowmore compact designs for indoor air treatment. This is clearly a verypromising technology which should be further investigated.

An additional common limitation to all biological air treatmentprocesses is the need to transfer contaminants into an aqueous phaseprior to their biodegradation, which is especially problematic in thecase of hydrophobic pollutants such as hexane. The addition of ahydrophobic organic phase into the bioreactors (two liquid phasepartitioning bioreactors) could significantly enhance the transfer ofthe pollutants to the microorganisms and thereby, their removal(Muñoz et al., 2007). Other possibilities include the addition ofactivated carbon or other adsorbents in combination with thebiological system. Such approaches should be investigated in thecase of indoor air treatment as they could also concentrate thecontaminants to levels suitable for growth.

7. Conclusions

Poor indoor air quality is a worldwide problem with tremendoushuman health and economical consequences. Although technologiesfor particle removal are rather well established, there are nowadays nosatisfactory methods for VOC control because removing indoor VOCsources or increasing ventilation rates is often not feasible oreconomical. There is therefore a need for designing specific airpurifying devices to clean and circulate the air inside affectedbuildings. Among the technologies potentially suitable for this purposeare biological systems relying upon the ability of plants and/ororganisms to detoxify organic compounds. However, a critical reviewof the existing literature in regards to biological and engineeringconstrains reveals numerous problems that must be solved beforebiologically-based air purifiers can be designed and implemented.

First, ourcurrent knowledgeonmicrobial kinetics and the thresholdsfor substrate uptake, consumption and gene expression raise seriousdoubt concerning the feasibility of microbial degradation of VOCs atindoor air concentrations. Yet, we also know biological systems (in a

broad sense) respond to indoor VOCs because of the linkages betweenVOC occurrence and SBS. In addition, there is experimental evidencethat VOCs can be biologically removed at indoor concentration even ifthe precise mechanisms are unknown. This apparent contradiction isperhaps explained by the fact that our current knowledge was derivedfrom studies conducted under conditions (single strains with singlesubstrate athighconcentration) irrelevant to the indoorair environment(diverse communities exposed to multiple substrates at low concentra-tions and direct pollutant uptake). Clearly, there is a need forfundamental research under indoor relevant conditions. This wouldnot only help to design and optimize indoor air biological purifiers butalso to solve growing environmental issues linked to trace contaminantsin water resources. One of the most interesting areas of research isperhaps the study of heterotroph–phototroph relationships such asthose observed in plants. This could explain why plants “appear” toremove VOCs at trace concentration when this hardly benefits them(from a detoxification point of view) and why microorganisms coulddegrade indoor VOCs and survive starvation.

Second, the design of biological air purifiers requires the develop-ment of new technologies for highly efficient pollutant transfer (fromair to the biological catalyst) in order to allow high volumetrictreatment flows while maintaining high treatment efficiencies.Current biological purifiers have shown some potential but are alllimited by their low treatment capacity. Solutions could be foundamong technologies (i.e. membrane bioreactors) that liberate from theneed to transfer VOCs to an aqueous phase. This opens interestingpossibilities for cross-disciplinary research initiatives.

Finally, as IAQ is linked to the presence of pollutants other thanVOCsand as biologicalmethodsmight always be limited in the cases of poorlysoluble or recalcitrant substances, there is a need to develop combinedphysicochemical–biological methods. This is especially necessary toeliminate potential nuisances from the biological purifier itself.

Acknowledgments

The “Université de Pau et de Pays de l'Adour” is gratefullyacknowledged for supporting the visit of DrGuieysse at its “Laboratoirede Thermique, Energétique et Procédés”.Wefinallywould like to thanksupports from the French–Mexican exchange program PCP 07-05“Evaluation of biofiltration supports for indoor air purification” (Etudedes performances d'un nouveau support de biofiltration appliqué à ladépollution de l'air intérieur) and from the Spanish Ministry ofEducation and Science (RYC-2007-01667 contract).

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